Electroencephalography (EEG) is a noninvasive measurement technique of the electrical potentials over the scalp, aiming to reconstruct the underlying primal electrical activity of the brain cortex. Due to the high temporal resolution of EEG, it is a valuable tool both for diagnosis of neural diseases (epilepsy being a well-known example) and for research.
On the other hand, conventional EEG measurement is capable of only low spatial resolution. One reason for this low spatial resolution is the unsolved complexity of electrical conductivity distribution within the head. Another reason is the influence of the skull on the propagation of the electrical signal from the neural sources to the scalp: The skull effectively acts as a spatial low-pass filter. This limitation restricts the number of meaningful EEG electrodes that can be distributed over the skull to about 200, which is much lower than the number of possible neural sources (about 10,000). This enormous difference between neural sources and effective measurement points makes the process of reconstruction of the cortical electrical activity from the EEG measurements severely ill-posed. The complexity of the cortical activations also gives rise to electrical noise, which can bury the electrical activity of interest and requires extensive averaging to overcome.
Recently, a number of techniques have emerged that intentionally perturb the electrical signal resulting from cortical activity, with the aim of improving the reconstruction process. For example, Helgason et al. describe an acousto-electric technique (AET) for current density imaging in “Application of acoustic-electric interaction for neuro-muscular activity mapping: A review,” European Journal of Translational Myology 24:4 (2015). In this technique, focused ultrasound is used to perturb locally the conductivity of the neural medium, giving potentially new information on the EEG signal.
As another example, Roth et al. describe magneto-acoustic imaging (MAI) of bioelectric currents in “The movement of a nerve in a magnetic field: application to MRI Lorentz effect imaging,” Medical & biological engineering & computing 52:5 (2014), pages 491-498. The goal in this technique is to measure the neural activity directly, employing the Lorentz force originating due to the magnetic field at the location of the electrical activity. This force induces the motion of the tissue that could be measured either by MRI or by measuring the acoustic field it induces.